Thermal Pressurant

ABSTRACT

The presently disclosed technology relates to using a combustion/decomposition heater fed by a working fluid stored within a storage tank to thermally pressurize the storage tank. The thermal pressurization may be used to maintain a desired pressure within the storage tank, even as the working fluid within the storage tank is drawn down. Further, a feedback mechanism may also be incorporated that varies the thermal energy added to the working fluid within the storage tank to maintain the desired pressure within the storage tank.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of priority to U.S. Provisional Patent Application No. 61/592,128, entitled “Thermal Pressurant” and filed on 30 Jan. 2012, which is specifically incorporated by reference herein for all that it discloses or teaches.

BACKGROUND

Thruster or rocket engines utilize combustion and/or decomposition of stored fuel and oxidizer to produce thrust. Other engines utilize combustion and/or decomposition of stored fuel and oxidizer to produce other types of work. The fuel and oxidizer may be stored separately and combined prior to combustion (e.g., a bipropellant) or premixed and stored prior to combustion (e.g., a monopropellant). Many fuels, oxidizers, and monopropellants (hereinafter working fluids) are stored at a pressure and temperature that allows the working fluid to exist in both liquid and gaseous phases within a storage tank. However, as the working fluid within the storage tank is consumed, pressure within the storage tank drops and gaseous-phase working fluid is boiled off from the liquid-phase working fluid. This gaseous-phase working fluid displaces the working fluid that is withdrawn from the storage tank to fill the constant volume of the storage tank. During the liquid boil-off process, the liquid cools due to forced evaporation as the tank pressure falls. This cooling effect decreases the vapor pressure of the liquid and the corresponding tank pressure over time. In many implementations, reduced storage tank pressure is undesirable for providing a consistent supply of working fluid (e.g., fuel to a thruster or other work-generating combustion engine). The reduced tank pressure may also result in lower performance of a component utilizing the working fluid. For example, reduced magnitude of useful output (e.g., power or propulsive thrust for a given component size) and/or reduced fuel economy (e.g., useful output divided by mass of working fluid consumed).

A portion of the work-generating engine may be located adjacent or surrounded by the storage tank. Waste heat from the work-generating combustion engine is transmitted to the storage tank as thermal energy. The thermal energy increases the temperature of the working fluid within the storage tank and vaporizes liquid-phase working fluid to gaseous-phase working fluid to counteract a temperature decrease within the storage tank caused by the discharge of working fluid from the storage tank. If the work-generating combustion engine is not adjacent or surrounded by the storage tank, a heat exchanger may be used to transfer thermal energy from the combustion engine to the storage tank.

These techniques of heating the working fluid within the storage tank suffer from poor control because they are dependent on the operation and operating temperature of the work-generating combustion engine.

SUMMARY

Implementations described and claimed herein address the foregoing problems by providing a method comprising extracting a first quantity of working fluid from a first outlet of a storage tank, exothermically reacting the extracted first quantity of working fluid to generate thermal energy, and transferring a majority of the generated thermal energy into working fluid remaining within the storage tank.

Implementations described and claimed herein further address the foregoing problems by providing a storage tank comprising a first outlet through which a first quantity of working fluid is extracted from the storage tank, and a heater that exothermically reacts the extracted working fluid generating thermal energy, wherein the heater further transfers a majority of the thermal energy into working fluid remaining within the storage tank.

Implementations described and claimed herein still further address the foregoing problems by providing a system comprising a working fluid storage tank with a first outlet and a second outlet, a heater fluidly connected to the first outlet that exothermically reacts working fluid discharged via the first outlet to generate thermal energy, wherein the heater further transfers a majority of the generated thermal energy into working fluid remaining within the storage tank, and an engine fluidly connected to the second outlet that exothermically reacts working fluid discharged via the second outlet to generate one or both of one or both of work and momentum.

Other implementations are also described and recited herein.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a cross-sectional perspective view of an example vessel utilizing a thermally pressurized storage tank with a dedicated combustion/decomposition heater.

FIG. 2 is a cross-sectional elevation view of an example thermally pressurized storage tank with a dedicated external combustion/decomposition heater.

FIG. 3 is a cross-sectional elevation view of an example thermally pressurized storage tank with a dedicated internal/external combustion/decomposition heater.

FIG. 4 is a cross-sectional elevation view of an example thermally pressurized storage tank and associated engine with a dedicated combustion/decomposition heater.

FIG. 5 is an example pressure-temperature phase diagram illustrating liquid and gaseous phases of a working fluid in an example thermally pressurized storage tank.

FIG. 6 is a graph that illustrates storage tank pressure and liquid-phase working fluid temperature as a function of liquid mass fraction in an example storage tank with no external thermal energy added.

FIG. 7 illustrates example operations for thermally pressurizing a storage tank using a dedicated combustion/decomposition heater.

DETAILED DESCRIPTIONS

Two-phase working fluids, which are typically part liquid-phase and part gaseous-phase fluids, may be challenging to regulate compared to pure liquid-phase or pure gaseous-phase working fluids. For example, the saturated liquid portion of two-phase working fluids flowing through a pump may readily cavitate, which can cause variations in mass flow rate of the working fluid through the pump and potential damage to the pump. Further, variations in gas/liquid ratio of a two-phase working fluid may cause significant variations in pressure drop through a fixed flow path. Still further, as the temperature and density of a liquid-phase working fluid changes, the conditions under which the liquid-phase vaporizes changes in the presence of a pressure drop. This introduces further uncertainty in a two-phase working fluid system.

The presently disclosed technology regulates a two-phase working fluid by controlling pressure and temperature conditions inside a storage vessel of the two-phase working fluid. If the working fluid has a high vapor pressure relative to an external environment into which the working fluid is ultimately expelled, the working fluid can generate sufficient pressure to adequately discharge from the storage vessel without supplemental pumping. In other implementations, pumping is required to generate sufficient discharge pressure.

As temperature of the two-phase working fluid increases, the working fluid vapor pressure increases. As a result, the temperature of the warmest liquid-phase element inside a storage vessel typically controls the pressure of the working fluid in the storage vessel. Furthermore, the pressure and temperature condition inside the storage-vessel containing the working fluid affects the gaseous-phase and liquid-phase densities of the working fluid. Since pressure of the working fluid is primarily controlled by the liquid-phase temperature, maintaining a desired temperature of the liquid-phase working fluid in the storage vessel is one way of precisely regulating fluid flow from the storage vessel.

Without an external heat source, evaporation of the liquid-phase working fluid may cause the temperature of the liquid-phase working fluid to drop, which in turn may cause the vapor pressure of the working fluid and corresponding tank pressure to drop. To maintain a tank pressure by maintaining a desired liquid-phase fluid temperature within a constant volume two-phase fluid undergoing mass loss, thermal energy may be added to allow evaporation of the liquid-phase into gaseous-phase without the overall liquid-phase temperature decreasing. This thermal energy can be derived from a dedicated heater that consumes a portion of the working fluid and releases heat through exothermic chemical reaction(s) of the consumed working fluid.

FIG. 1 is a cross-sectional perspective view of an example vessel 100 utilizing a thermally pressurized storage tank 102 with a dedicated combustion/decomposition heater 114. The vessel 100 could be a rocket, thruster, or other engine that converts combustion/composition of propellant into thrust. The storage tank 102 contains a propellant that is discharged via an outlet 104 into lines 106. One or more valves (e.g., valve 108) and other equipment may also be located at the discharge of the tank 102. The lines 106 lead to an ignition interface 110 where the propellant is ignited and propelled out of an expansion nozzle 112. Due to conservation of energy and mass, the discharge of the combusted propellant out of the nozzle 112 from right to left causes the vessel 100 to be propelled from left to right in FIG. 1. In one implementation, the vessel 100 is a part of a larger vessel (e.g., vessel 100 is a thruster on a space station). In other implementations, the propellant discharged via outlet 104 may be used in another work-producing and/or gas-generating system.

The heater 114 includes lines 116 connecting the storage tank 102 to a control valve 118 that regulate the flow of propellant (illustrated by arrow 117) to a combustion/decomposition chamber 120 where the regulated propellant flow is combusted and/or decomposed. In various implementations, the combustion/decomposition chamber 120 is spark ignited or compression ignited. Further, the combustion/decomposition chamber 120 may utilize a catalyst bed to facilitate combustion/decomposition. Further, the size of the combustion/decomposition chamber 120 may vary as compared to a corresponding heat exchanger 122.

The heat exchanger 122 transfers thermal energy generated within the combustion/decomposition chamber 120 to the propellant within the storage tank 102, as evidenced by wavy arrows. In some implementations, the heat exchanger 122 is a micro-fluidic heat exchanger, which utilizes a heat-exchanging matrix of micro-fluidic passages to efficiently transfer heat from combustion/decomposition exhaust gasses to the propellant through conductive contact with the storage tank 102. In various implementations, the heater 114 transfers a majority (i.e., greater than about 50%) of the thermal energy generated by the heater 114 to the working fluid within the storage tank 102. In other implementations, the transferring operation 630 transfers greater than about 90% of the thermal energy generated by the heater 114 to the working fluid within the storage tank 102.

The transfer of thermal energy to the storage tank 102 may vaporize some of the liquid-phase propellant to a gaseous-phase as depicted by bubbles 124. Combusted gasses 123 generated within the combustion chamber 120 are exhausted via exhaust pipe 121 in a direction or manner that does not damage nearby equipment (e.g., exhausted out of the vessel 100). Further, the exhaust gas 123 momentum may be used for supplemental heat generation and/or thrusting of the vessel 100. Still further, the momentum of the exhaust gases 123 may be directed to a nozzle mounted on a gimbal (not shown) and used for directional control of the vessel 100. Further yet, the exhaust gases may be used in a decomposition engine (e.g., N₂O may be decomposed into O₂ and N₂) to generate more energy.

The presently disclosed technology works particularly well with exothermically reacting fluids that have a high vapor pressure (e.g., typically significantly greater than 101 kPa at 25° C.). As a result, the exothermically reacting fluids are substantially pressurized by the vapor of the propellant within the storage tank 102. However, the presently disclosed technology may also work with exothermically reacting fluids that have a low vapor pressure (e.g., typically less than 101 kPa at 25° C.) and either operate well in a low pressure feed systems or are pressurized within the storage tank using a pressurizing gas (e.g., nitrogen). Example propellants that may be used as the propellant in the presently disclosed technology include nitrous oxide, nitrous oxide fuel blends, hydrazine, ammonia, and bipropellant combinations of fuels and oxidizers such as liquid oxygen and a fuel (e.g., kerosene, methane, ethane, ethylene, propane, butane, etc.), nitrous oxide and a fuel (e.g., kerosene, methane, ethane, ethylene, propane, butane, etc.), etc.

The relative location of the heater 114 and lines 106 leading to the ignition interface 110 and nozzle 112 may vary significantly from that depicted in FIG. 1, so long as thermal energy from the heater 114 is ultimately transferred to the storage tank 102. More specifically, the lines 116 feeding the heater 114 may draw liquid-phase or gaseous-phase propellant for combustion/decomposition. Further, the heater 114 may provide thermal energy to areas of the storage tank 102 other that that depicted in FIG. 1. Still further, the lines 106 leading to the ignition interface 110 and nozzle 112 may lead from areas of the storage tank 102 other that that depicted in FIG. 1.

In one implementation, the heat exchanger 122 is physically located near the outlet 104 of the storage tank 102 so that the thermal energy is added primarily to the liquid-phase propellant during a majority of fill states of the storage tank 102. Since the liquid phase propellant is likely to be more thermally conductive than the gaseous-phase propellant, adding thermal energy to the liquid-phase propellant is less likely to create a local hot spot that exceeds a design limitation of the storage tank 102 or be a potential point of ignition of the propellant within the storage tank 102.

The propellant may be a monopropellant or bipropellant and may include one or more fuels and oxidizers. Fuels, oxidizers, monopropellants, bipropellants, and other stored fluids are all referred to herein as working fluids. In other implementations, multiple storage tanks may be utilized store multiple fuels, oxidizers, or other fluids. For example, tank 102 may store a fuel and another storage tank may store an oxidizer. Lines combine the fuel and oxidizer upstream of the ignition interface 110 so that the mixed fuel/oxidizer may be ignited at the ignition interface 110 and discharged via the nozzle 112 and/or consumed by the heater 114. Further, where the tank 102 stores a fuel, the oxidizer may be an ambient fluid (e.g., atmospheric air). The tank 102 is depicted as ellipsoidal but could be any volume-enclosing shape (e.g., platonic, quadric, or any combination thereof).

FIG. 2 is a cross-sectional elevation view of an example thermally pressurized storage tank 202 with a dedicated external combustion/decomposition heater 214. The storage tank 202 is equipped with an outlet 204 with a control valve 208 that directs working fluid 232 to a work-generating engine (not shown), as illustrated by arrow 226. In other implementations, the outlet 204 and the control valve 208 directs the working fluid 232 to a thruster, gas-generator, or heater (not shown). The working fluid 232 flows from the outlet 204 of the storage tank 202, through the valve 208 (as illustrated by arrow 226), and into the engine, where energy is extracted from the working fluid 232. The control valve 208 may vary the flow rate of the working fluid 232 out of the storage tank 202 to provide a desired energy output per unit of time from the engine. In other implementations, the working fluid 232 flowing through the control valve 208 may be used to generate gas for purposes other than generating work or thrust (e.g., inflating a membrane).

The heater 214 includes lines 216 connecting an outlet 230 of the storage tank 202 to a control valve 218 that regulates the flow of working fluid 234 to the heater 214. More specifically, the working fluid 234 is combusted and/or decomposed in a combustion/decomposition chamber 220 and a heat exchanger 222 transfers heat generated within the combustion/decomposition chamber 220 to the working fluid 232 within the storage tank 202, as evidenced by wavy arrows. The transfer of thermal energy to the storage tank 202 vaporizes some of the liquid-phase working fluid 232 to a gaseous-phase as depicted by bubbles 224. Combustion/decomposition gasses generated within the combustion chamber 220 are exhausted (as illustrated by arrows 228) in a direction or manner that does not damage nearby equipment. In other implementations, the exhaust gases are used for supplemental heat generation, thrusting, and/or additional combustion/decomposition.

The heater 214 regulates pressure within the storage tank 202 by adding thermal energy to the working fluid 232 within the storage tank 202. Since the storage tank 202 is at a pressure-temperature equilibrium state where both liquid-phase working fluid 232 and gaseous-phase working fluid 234 is present, the additional thermal energy converts some of the liquid-phase working fluid 232 to a gaseous-phase. Since gaseous-phase working fluid occupies more volume than liquid-phase working fluid in the same temperature and pressure conditions, the pressure within the storage tank 202 increases because the storage tank volume remains the same. Further, the temperature of the working fluid 232 in the vicinity of the heat exchanger 222 increases.

The flow rate of the working fluid 234 through the control value 218 may be regulated to provide sufficient thermal energy to the storage tank 202 to maintain a desired pressure within the storage tank 202 and compensate for lost pressure caused by the discharge of working fluid 232, 234 via the outlets 204, 230, respectively. In one implementation, combustion/decomposition of about 1.5% of the extracted working fluid mass flow rate exiting outlet 204 is sufficient to maintain a desired pressure within the storage tank 202. The remaining about 98.5% of the mass flow rate exits the storage tank 202 via the outlet 204. Further, the storage tank 202 may be equipped with one or more temperature and/or pressure measuring devices (e.g., gauges and/or transducers) (not shown) and a feedback loop that actuates the control value 218 to maintain and/or attain desired conditions within the storage tank 202 and/or at the outlets 204, 230. See e.g., FIG. 4.

In one implementation, when the valve 208 is opened to a lower pressure area and the working fluid 232 exits the tank 202 as illustrated by the arrow 226, the working fluid 232 may partially or entirely enter a gaseous-phase as it exits the storage tank 202. The phase change from a liquid-phase to a gaseous-phase (i.e., flash evaporation) of part or all of the working fluid 232 exiting the storage tank 202 at the outlet 204 absorbs thermal energy from the working fluid 232. This absorption of thermal energy may decrease the temperature of the working fluid 232 within the storage tank 202 (especially locally in the vicinity of the outlet 204). The combustion heater 214 can further counteract the temperature decrease within the storage tank 202 caused by the flash evaporation of the working fluid 232 exiting the storage tank 202, especially if the combustion heater 214 is located near the outlet 204, as illustrated in FIG. 2.

The liquid-phase working fluid 232 is depicted as settling at the bottom of the storage tank 202 near the outlet 204 and the gaseous-phase working fluid 234 rising to the top of the storage tank 202. This is due to the gaseous-phase working fluid 234 having substantially less density that the liquid phase working fluid 232 and the storage tank 202 being under a force directed in the general direction of arrow 226 (e.g., a gravitational force or an inertial force opposite upward acceleration of the storage tank 202). Further, in the absence of any phase-separating forces on the storage tank 202, prior phase-separation of the working fluid 232, 234 achieved when the storage tank 202 was previously under the influence of one or more forces may be generally maintained indefinitely. For example, acceleration of a spacecraft out of the Earth's atmosphere may phase-separate the working fluid 232, 234 and travel through space using the spacecraft's momentum and no additional thrust should maintain the phase separate of the working fluid indefinitely.

In other implementations, the phase-separation of the working fluid 232, 234 depicted in FIG. 2 may be reversed, or oriented along any other axis. Further, the phase-separation of the working fluid 232, 234 may vary over time. Still further, in the absence of any consistent phase-separating forces on the storage tank 202, pockets of liquid-phase working fluid 232 may be evenly distributed throughout the storage tank 202, or vice versa.

Since the heater 214 is located near the outlet 204 and the outlet 204 preferentially draws liquid-phase working fluid 232, the heater 214 transfers thermal energy primarily to the liquid-phase working fluid 232 in most states of filling the storage tank 202 (excepting nearly or fully empty fill states of the storage tank 202, e.g., less than 10% or 5% filled). It may be desired to transfer thermal energy primarily to the liquid-phase working fluid 232 because liquid-phase working fluids generally have greater thermal conductivity that gaseous-phase states of the same working fluids. As a result, thermal energy transferred to the liquid-phase working fluid 232 is more readily distributed throughout the liquid-phase working fluid 232 and the likelihood that a hot spot caused by the heater 214 exceeds design limitations of the storage tank 202 or working fluids 232, 234 therein is reduced.

In other implementations, the heater 214 is not located near the outlet 204 and/or not located primarily adjacent to the liquid-phase working fluid 232. In these implementations, the maximum output of the heater 214 may be calibrated such that it is insufficient to create a hot spot that exceeds design limitations of the storage tank 202 or working fluid 232, 234, regardless of whether liquid-phase working fluid 232, gaseous-phase working fluid 234 or a combination of phases is adjacent the heater 214 in the storage tank 202.

The outlet 230 is located approximately opposite (e.g., 170-190 degrees) of the heater 214 on the storage tank 202. As a result, the outlet 230 primarily draws gaseous-phase working fluid 234 in most states of filling the storage tank 202 (excepting nearly or fully filled fill states of the storage tank 202, e.g., greater than 90% or 95% filled). It may be desired to draw exclusively or nearly exclusively gaseous-phase working fluid 234 out of the outlet 230 because the heater 214 is designed to operate exclusively or preferentially with gaseous-phase working fluid 234. Further, even if the heater 214 operates with either gaseous-phase working fluid 234 or liquid-phase working fluid 232, locating the outlet 230 such that primarily gaseous-phase working fluid 234 is drawn out of the outlet 230 may prevent wide fluctuations in the output of the heater 214 due to the substantially different densities of the liquid-phase working fluid 232 and the gaseous-phase working fluid 234.

In other implementations, the outlet 230 is located near the heater 214 rather than opposite the heater 214 on the storage tank 202. Further, the outlet 230 may be located such that it primarily draws liquid-phase working fluid 232 rather than gaseous-phase working fluid 234 in most states of filling the storage tank 202.

FIG. 3 is a cross-sectional elevation view of an example thermally pressurized storage tank 302 with a dedicated internal/external combustion/decomposition heater 314. The storage tank 302 is equipped with an outlet 304 with a control valve 308 that directs working fluid 332 to an engine (not shown). The working fluid 332 flows from the outlet 304 of the storage tank 302, through the valve 308 (as illustrated by arrow 326), and into the engine, where work, thrust, or other energy is extracted from the working fluid 332. The control valve 308 may vary the flow rate of the working fluid 332 out of the storage tank 302 to provide a desired work, thrust, or other energy output from the engine.

The internal/external heater 314 includes lines 316 connecting an outlet 330 of the storage tank 302 to a control valve 318 that regulates the flow of working fluid 334 to the heater 314. More specifically, the working fluid 334 is combusted and/or decomposed in a combustion/decomposition chamber 320 and a heat exchanger 322 transfers heat generated within the combustion/decomposition chamber 320 to the working fluid 332 within the storage tank 202, as evidenced by wavy arrows. The transfer of thermal energy to the storage tank 302 vaporizes some of the liquid-phase working fluid 332 to gaseous-phase working fluid 334 as depicted by bubbles 324. Combusted/decomposed gasses generated within the combustion/decomposition chamber 320 are exhausted (as illustrated by arrows 328) in a direction or manner that does not damage nearby equipment. In other implementations, the exhaust gases are used for supplemental heat generation, thrusting, and/or additional combustion/decomposition.

The internal/external heater 314 is partially encompassed by the storage tank 302. Thus a greater amount of the surface area of the heat exchanger 322 is in contact with the working fluid 332 within the storage tank 302. This allows the internal/external heater 314 to transmit more thermal energy in a given amount of time to the working fluid 332 within the storage tank 302 than a purely external heater. Various implementations of heat exchangers may be purely external to the storage tank 302 (see e.g., heat exchanger 222 of FIG. 2), partially internal and external (see e.g., heat exchanger 322 of FIG. 3), and/or purely internal where the entire heat exchanger is encompassed by the storage tank 302 (not shown). The choice of heat exchanger location relative to the storage tank 302 depends on the surface area available for heat transfer and the surface area required to transfer a desired quantity of thermal energy to the working fluid 332 within the storage tank 302, for example.

The flow rate of the working fluid 334 through the control value 318 may be regulated to provide sufficient thermal energy to the storage tank 302 to maintain a desired pressure within the storage tank 302 and compensate for lost pressure caused by the discharge of working fluid 332, 334 via the outlets 304, 330, respectively. Further, the storage tank 302 may be equipped with one or more temperature and/or pressure measuring devices (e.g., gauges and/or transducers) (not shown) and a feedback loop that actuates the control value 318 to maintain and/or attain desired conditions within the storage tank 302 and/or at the outlets 304, 330. See e.g., FIG. 4.

The liquid-phase working fluid 332 is depicted as settling at the bottom of the storage tank 302 near the outlet 304 and the gaseous-phase working fluid 334 rising to the top of the storage tank 302. In other implementations, the phase-separation of the working fluid 332, 334 depicted in FIG. 3 may be reversed, or oriented along any other axis. Further, the phase-separation of the working fluid 332, 334 may vary over time. Still further, in the absence of any consistent phase-separating forces on the storage tank 302, pockets of liquid-phase working fluid 332 may be evenly distributed throughout the storage tank 302, or vice versa.

The heater 314 may be located anywhere on or within the storage tank 302 walls. For example, heater 214 is located near the outlet 204 of FIG. 2. Heater 314 is located away from outlet 304 of FIG. 3. The maximum output of the heater 314 may be calibrated such that it is insufficient to create a hot spot that exceeds design limitations of the storage tank 302 or working fluid 332, 334, regardless of whether liquid-phase working fluid 332, gaseous-phase working fluid 334 or a combination of phases is adjacent the heater 314 in the storage tank 302.

FIG. 4 is a cross-sectional elevation view of an example thermally pressurized storage tank 402 and associated engine 412 with a dedicated combustion/decomposition heater 414. The storage tank 402 is equipped with an outlet 404 with a control valve 408 that directs working fluid 432 to the engine 412, where the working fluid 432 is to be utilized. The working fluid 432 flows from the outlet 404 of the storage tank 402, through the valve 408, and into the engine 412, where work (or other useable energy) is extracted from the working fluid 432. In other implementations, the engine 412 may generate gas to inflate a membrane or generate pressure.

The control valve 408 may be designed for flow regulation to provide a desired work, energy, or gas generation output from the engine 412. In other implementations, the control valve 408 functions merely as an on/off valve and the working fluid 432 flow rate from the outlet 404 is primarily controlled by the pressure/temperature conditions within the storage tank 402. In yet other implementations, the control valve 408 and the heater 414 work in concert to produce a set of desired pressure/temperature conditions within the storage tank 402 and provide flow regulation through the control valve 408.

The heater 414 includes lines 416 connecting an outlet 430 of the storage tank 402 to a control valve 418 that regulates the flow of working fluid 434 to the heater 414. More specifically, the working fluid 434 is combusted and/or decomposed in a combustion/decomposition chamber 420 and a heat exchanger 422 transfers heat generated within the combustion/decomposition chamber 420 to the working fluid 432 within the storage tank 402, as evidenced by wavy arrows. The transfer of thermal energy to the storage tank 402 vaporizes some of the liquid-phase working fluid 432 to a gaseous-phase working fluid 434 as depicted by bubbles 424. In some implementations, the storage tank 402 of FIG. 4 may include additional features (e.g., fins) that facilitate the transfer of thermal energy to the working fluid 432.

The liquid-phase working fluid 432 is depicted as settling at the bottom of the storage tank 402 near the outlet 404 and the gaseous-phase working fluid 434 rising to the top of the storage tank 402. In other implementations, the phase-separation of the working fluid 432, 434 depicted in FIG. 4 may be reversed, or oriented along any other axis. Further, the phase-separation of the working fluid 432, 434 may vary over time. Still further, in the absence of any consistent phase-separating forces on the storage tank 402, pockets of liquid-phase working fluid 432 may be evenly distributed throughout the storage tank 402, or vice versa.

The heater 414 may be located anywhere on or within the storage tank 402 walls. For example, heater 214 is located near the outlet 204 of FIG. 2. Heater 414 is located away from outlet 404 of FIG. 4. The maximum output of the heater 414 may be calibrated such that it is insufficient to create a hot spot that exceeds design limitations of the storage tank 402 or working fluid 432, 434, regardless of whether liquid-phase working fluid 432, gaseous-phase working fluid 434 or a combination of phases is adjacent the heater 414 in the storage tank 402.

The engine 412 may include a combustion and/or decomposition chamber to combust and/or decompose the working fluid 432. In one implementation, the engine 412 is a rocket engine used to generate thrust. In another implementation, the engine 412 is capable of generating useful work. In yet another implementation, the engine 412 is a gas generator. The gas generator may be used for providing gas to fill membranes or pressurizing a device. The gas generator may also be used to provide gas at a desired flow rate to support further chemical reactions. For example, the gas generator may be used to generate breathing air for biological reactions. The engine 412 may also represent other equipment that may use the working fluid 432 discharged from the storage tank 402.

The flow rate of the working fluid 434 through the control value 418 may be regulated to provide sufficient thermal energy to the storage tank 402 to achieve and/or maintain desired pressure and/or temperature conditions within the storage tank 402 and compensate for lost pressure and/or temperature caused by the discharge of working fluid 432, 434 via the outlets 404, 430. Further, although the pressure throughout the storage tank 402 may be approximately uniform, the temperature distribution inside the storage tank 402 may not be uniform based on the local additional of thermal energy to the working fluid 432 near the heater 414.

In an example implementation, a storage tank pressure sensor 436 (e.g., a gauge or a pressure transducer) monitors the storage tank pressure and in a feedback control system varies the flow rate of the working fluid 434 (and thus the thermal energy transferred to the working fluid 432 within the storage tank 402) via the control value 418 to maintain or attain a desired pressure within the storage tank 402. In some implementations, the desired pressure is a range that is governed by the design pressure of the storage tank 402, pressure conditions external to the storage tank 402, desired pressure at the outlet 404, and desired pressure at the engine, for example. Further, the storage tank pressure may be configured to follow a desired curve as a function of time or state of fill of the storage tank 402. For example, the storage pressure may be held relatively constant until the storage tank 402 is nearly empty, and then the pressure is allowed to drop off rapidly.

In another example implementation, a heat exchanger temperature sensor 438 is located near the heat exchanger 422, on or within the storage tank 402. The temperature sensor 438 monitors the temperature of the storage tank 402 and/or working fluid 432 within the storage tank 402 in the vicinity of the heat exchanger 422 and in a feedback control system varies the flow rate of the working fluid 434 (and thus the heat transferred to the working fluid 432 within the storage tank 402) via the control valve 418 to maintain or attain a desired temperature of the storage tank 402 and/or working fluid 432 within the storage tank 402 in the vicinity of the heat exchanger 422. In some implementations, the desired temperature is a range that is governed by a maximum and/or minimum design temperature of the storage tank 402 and/or an auto-ignition temperature or decomposition temperature of the working fluid 432 (e.g., less than 200° C.).

Further, a temporal spike in the temperature of the storage tank 402 in the vicinity of the heat exchanger 422 may indicate that the working fluid within the storage tank 402 in the vicinity of the heat exchanger 422 is no longer the liquid-phase working fluid 432, but the gaseous-phase working fluid 434. This may be caused by the reduced thermal conductivity of the gaseous-phase working fluid 434 as compared to the liquid-phase working fluid 432. One or both control vales 418, 408 may be actuated to reduce the working fluid flow rate out of the storage tank 402 because the working fluid 432 within the storage tank 402 is getting low or is completely consumed. In another implementation, the storage tank 402 is equipped with a level sensor 440 that directly measures the liquid-phase working fluid 432 level within the storage tank 402 and may provide feedback control to one or both of control vales 418, 408.

In yet another implementation, a discharge temperature sensor 442 is located near or within the outlet 404 of the storage tank 402. The temperature sensor 442 monitors the temperature of the working fluid 432 leaving the storage tank 402 and in a feedback control system varies the flow rate of the working fluid 432, 434 through one or both of the control valves 408, 418 to maintain or attain a desired temperature of the working fluid leaving the storage tank 402. In some implementations, the desired temperature is a range that is governed by a minimum design temperature of the storage tank 402 (or a freezing point of the working fluid, e.g., less than −100° C.), a minimum temperature of the working fluid 432 for effective ignition for combustion/decomposition within the engine 412, and/or a maximum variation between two or more measured temperatures of the working fluid within the storage tank 402 (e.g., a maximum variation between the measured temperature at sensor 438 and sensor 442). An indication that the temperature variation is too large may reduce the working fluid flow rate through either or both of valves 408, 416 or trigger an agitation mechanism for the storage tank 402 to work to equilibrate the working fluid temperature.

Additional pressure, temperature, and/or flow rate sensors may be located at the outlet 404 to determine feeding and operating conditions of the engine 412. However, potential phase changes (liquid-to-gaseous phase or gaseous-to-liquid phase, resulting in a “frothy” liquid), pressure changes, and temperature changes as the working fluid travels from the storage tank 402 to the engine 412, may not be detected by sensors at the outlet 404 and thus sensors at the outlet 404 may not give reliable information about the feeding and operating conditions of the engine 412.

A pressure sensor 444 and a temperature sensor 446 may be located adjacent to or within the engine 412, specifically measuring conditions within a decomposition/combustion chamber of the engine 412 or conditions downstream of the engine 412. Further, the flow rate of the working fluid 432 to the engine 412 may be calculated using the measured pressure and temperature conditions within the decomposition/combustion chamber of the engine 412. Further, the flow rate of the working fluid 432 through the control value 418 may be regulated to provide the working fluid 432 to the decomposition/combustion chamber of the engine 412 at a desired pressure. Still further, the desired pressure may be configured to follow a desired curve as a function of time or state of fill of the storage tank 402. For example, the decomposition/combustion chamber pressure may be held relatively constant until the storage tank 402 is nearly empty, and then the pressure is allowed to drop off rapidly or at a desired rate. The flow rate of the working fluid 434 through the control value 418 may further be used to vary pressure within the decomposition/combustion chamber of the component 412 in order to selectively throttle the engine 412.

Still further, the control valve 418 may be directly liked to the control valve 408. For example, the control valve 418 may open when the control valve 408 is opened and close when the control valve 408 is closed. The opening of the control valve 418 may be proportional to the opening of the control valve 408 or the control valve 418 may operate only in on/off configurations. There may also be a time delay associated with opening and/or closing the control valve 418 when the control valve 408 is opened and/or closed (e.g., the control valve 408 remains open a specific period of time after the control valve 418 is closed). The control valves 408, 418 may operate with any one or more of the aforementioned control schemes, along with additional control schemes.

FIG. 5 is an example pressure-temperature phase diagram 500 illustrating liquid and gaseous phases of a working fluid in an example thermally pressurized storage tank. The phase diagram 500 illustrates temperature within the storage tank on an x-axis and pressure within the storage tank on a y-axis. Coexistence curve 520 separates the liquid-phase from the gaseous-phase of the working fluid. When the pressure and temperature of the working fluid lies on the coexistence curve 520, both liquid-phase and gaseous-phase working fluid may be present within the storage tank. Further, the coexistence curve 520 is illustrated between a triple point 522 and a critical point 524. The triple point 522 is the temperature and pressure at which three phases (e.g., gas, liquid, and solid) of the working fluid coexist in thermodynamic equilibrium. The critical point 524 specifies the conditions (e.g., temperature (T_(cr)), pressure (P_(cr)), and sometimes composition) at which a phase boundary ceases to exist. The coexistence curve 520 primarily illustrates the boundary conditions between liquid-phase and gaseous-phase working fluid.

Typically, as a storage tank containing both liquid-phase and gaseous-phase working fluid discharges working fluid, the temperature and pressure within the storage tank falls due to endothermic vaporization of liquid-phase working fluid into gaseous-phase working fluid to fill the fixed volume of the storage tank. A falling storage tank pressure may not be desirable for applications that utilize storage tank pressure to flow the working fluid to a point of utilization of the discharged working fluid. The heaters (see e.g., heater 214 of FIG. 2) disclosed herein are used to counteract the falling storage tank pressure by adding thermal energy back into the working fluid within the storage tank to maintain a relatively constant storage tank pressure so long as there is both liquid-phase and gaseous-phase working fluid within the storage tank. As soon as the last liquid-phase working fluid is boiled into a gaseous-phase, the storage tank pressure rapidly drops off as the storage tank becomes essentially empty.

For example, the storage tank may be at pressure P₁ and temperature T₁ (i.e., at point 526) at equilibrium and have working fluid intermixed within the storage tank in both a liquid-phase and gaseous-phase. When working fluid is discharged from the storage tank, the temperature within the storage tank drops (see temperature T₂) due to endothermic vaporization of liquid-phase working fluid into gaseous-phase working fluid to occupy the volume of discharge working fluid within the storage tank. As a result, pressure of the working fluid within the storage tank also drops (see pressure P₂) due to the temperature drop (see point 528) and the presence of both liquid-phase and gaseous-phase working fluid within the storage tank.

When a thermal energy input (e.g., via the heater 214 of FIG. 2) is applied to the working fluid, the local temperature of the working fluid near the heater rises back to T₁. This forces the liquid working fluid near the heater into a gaseous-phase. Further, as the liquid-phase working fluid near the heater evaporates, the pressure in the entire storage tank rises back to P₁ (see e.g., point 526). As a result, the heater is capable of maintaining the pressure within the storage tank between P₁ and P₂ and the temperature within the storage tank between T₁ and T₂, so long as there is both liquid-phase and gaseous-phase working fluid within the storage tank.

In other implementations, the heater is capable of maintaining the pressure within the storage tank at a singular point, so long as there is both liquid-phase and gaseous-phase working fluid within the storage tank. Still further, the heater may be operated on a closed-loop feedback system in order to maintain a desired pressure within the storage tank. Further yet, boiling of the liquid-phase working fluid near the heater causes the added thermal energy to rapidly distribute the thermal energy throughout the entire liquid-phase portion of the working fluid within the storage tank. This helps reduces the possibility of a hot-spot developing adjacent the heater by coupling thermal energy into a much larger thermal mass associated with the entire liquid volume rather than more localized volume. However, working fluids with a very low vapor pressure may not exhibit this enhancement in convective heat distribution through aggressive boiling. It should be noted that relatively low temperature boiling of a high vapor pressure fluid in an open tank caused by a localized heat source (e.g., heater 214 of FIG. 2) as described in detail herein augments heat transport throughout a relatively static fluid. This is distinct from inhibiting heat transfer into the working fluid like nucleate and/or film boiling typically do for flowing fluids in heat exchangers where convection is dominated by the convection associated with the velocity of the flowing working fluid.

FIG. 6 is a graph 600 that illustrates storage tank pressure and liquid-phase propellant temperature as a function of liquid mass fraction in an example storage tank with no external thermal energy added. The graph 600 assumes that the storage tank starts essentially full of liquid-phase working fluid (i.e., a liquid mass fraction of 1.0) at 30° C. and drops over time to essentially empty state (i.e., a liquid mass fraction of 0.0) as working fluid is discharged from the storage tank (as illustrated by arrow 601). Other implementations may start and/or end with a liquid mass fraction between 0.0 and 1.0.

As the liquid mass fraction drops from 1.0 to 0.0, both the storage tank pressure and the liquid-phase propellant temperature dramatically drop, especially as the storage tank nears a 0.0 liquid mass fraction. Further, as described above with regard to FIG. 5, the storage tank pressure and propellant temperature are closely related, especially when both liquid-phase and gaseous-phase propellant are present within the storage tank. This is illustrated by the tank pressure and liquid propellant temperature curves closely tracking one another. If thermal energy is added to the storage tank as the liquid mass fraction drops (e.g., via the heater 214 of FIG. 2), the reduction in tank pressure and liquid-phase propellant temperature can be counteracted and the storage tank can maintain a relatively constant pressure until the liquid-phase mass fraction nears 0.0, at which point the storage tank pressure will rapidly drop to zero.

FIG. 7 illustrates example operations 700 for thermally pressurizing a storage tank using a dedicated combustion/decomposition heater. A providing operation 705 provides a storage tank containing a working fluid at equilibrium in both liquid and gaseous phases. A first extracting operation 710 extracts working fluid from the storage tank via a first outlet. The extracted working fluid may be metered to an engine or other equipment that generates work or other useable energy, generates gas, or produces a fluid having desired chemical properties. A first generating operation 715 generates energy from decomposition and/or combustion of the working fluid extracted from the storage tank via the first outlet. Further, the generating operation 715 may produce gas of a desired pressure, flow rate, and/or chemistry. In one implementation, the decomposition/combustion occurs within the engine and the engine outputs the work (or energy). The energy may be in the form of thrust for a vessel, mechanical work (e.g., turning a crankshaft) for propelling a vehicle or generating electricity, and/or thermal energy for heat generation, for example. Operation 715 may also generate gas that could be used for filling or inflating operations or providing gas of a desired chemistry (e.g., breathing air). The first extracting and generating operations 710, 715 may repeat to feed the engine or other equipment and generate work, energy, and/or gas continuously or on demand. Further, the first extracting and generating operations 710, 715 may vary in terms of extraction rate and work, energy, and/or gas output rate over time.

A second extracting operation 720 extracts the working fluid from the storage tank via a second outlet. The working fluid extracted from the second outlet is separate from the working fluid extracted from the first outlet. In various implementations, the working fluid is extracted from the same or different general locations on the storage tank via the first and second outlets. In some implementations, the working fluid is extracted from the same outlet on the storage tank and then split into the first and second outlets downstream of the storage tank outlet. Further, the extracted working fluid may be in a liquid and/or gaseous phase. The working fluid extracted from the second outlet is fed (and may be metered) to a decomposition/combustion chamber of a heater. A second generating operation 725 generates thermal energy from decomposition and/or combustion of the working fluid extracted from the second outlet. The heater is specifically configured to extract thermal energy from decomposition and/or combustion of the working fluid extracted from the second outlet. The second extracting and generating operations 720, 725 may repeat to feed the heater and generate thermal energy continuously or on demand. Further, the second extracting and generating operations 720, 725 may vary in terms of extraction rate and thermal energy output rate over time.

A transferring operation 730 transfers the generated thermal energy to the storage tank to attain and/or maintain one or more desired storage tank conditions (e.g., pressure and/or temperature) while the working fluid is extracted from the first outlet and the second outlet. The transferring operation 730 may be accomplished via a heat exchanger (e.g., a micro-fluidic heat exchanger) that thermally couples the heater to the storage tank. The transferring operation 730 regulates pressure and/or temperature conditions within the storage tank by adding thermal energy to the working fluid remaining within the storage tank. This compensates for thermal energy loss via endothermic liquid-to-gaseous phase change of the working fluid remaining within the storage tank as working fluid is withdrawn. In one implementation, the transferring operation 730 transfers a majority (i.e., greater than about 50%) of the thermal energy generated by the heater to the working fluid within the storage tank. In other implementations, the transferring operation 730 transfers greater than about 90% of the thermal energy generated by the heater to the working fluid within the storage tank.

More specifically, the storage tank is at a pressure equilibrium state where both liquid-phase and gaseous-phase working fluid is present. Since gaseous-phase working fluid occupies more volume than liquid-phase working fluid in the same temperature and pressure conditions, the additional thermal energy converts some of the liquid-phase working fluid to gaseous-phase working fluid. This conversion displaces the volume of the working fluid that has been expelled from the tank through extracting operations 710, 720. Further, this process compensates for lost pressure, which would otherwise occur due to evaporative cooling of the liquid working fluid in the tank to provide this boil-off gas. In some implementations, thermal energy may be used to increase the pressure within the storage tank because the storage tank volume remains the same, all else being equal. This increased pressure compensates for lost pressure as the working fluid exits the storage tank via the extracting operations 710, 720.

A varying operation 735 varies the thermal energy transfer rate to maintain the one or more desired storage tank conditions as the total working fluid discharge rate changes. This may be accomplished by varying the working fluid discharge rate of the second extraction operation 720 responsive to the total working fluid discharge rate of the first and second extraction operations 710, 720 via a closed-loop feedback control system (see e.g., FIG. 4).

More specifically, the second extracting operation 720 may be metered to match the transferred thermal energy of the transferring operation 730 with the rate of thermal energy loss caused by the extraction operations 710, 720. One or more feedback circuits may be utilized to control the extraction operations 710, 720 (see e.g., FIG. 4). As a result, the pressure within the storage tank may be controlled regardless of the rate of extraction operations 710, 720. Still further, a set pressure within the storage tank may be targeted, achieved, and maintained using the above operations 700. Further, the operations 700 may be iteratively repeated and flow rates modified continuously to feed the engine and heater and attain/maintain desired pressure/temperature conditions within the storage tank.

The above specification, examples, and data provide a complete description of the structure and use of exemplary embodiments of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended. Furthermore, structural features of the different embodiments may be combined in yet another embodiment without departing from the recited claims. 

What is claimed is:
 1. A method comprising: extracting a first quantity of working fluid from a first outlet of a storage tank; exothermically reacting the extracted first quantity of working fluid to generate thermal energy; and transferring a majority of the generated thermal energy into working fluid remaining within the storage tank.
 2. The method of claim 1, further comprising: extracting a second quantity of working fluid from a second outlet of the storage tank; and exothermically reacting the second quantity of extracted working fluid to provide one or both of work and momentum.
 3. The method of claim 2, further comprising: varying a rate the first quantity of working fluid is extracted from the storage tank responsive to a combined rate the first and the second quantities of working fluid are extracted from the storage tank.
 4. The method of claim 3, further comprising: varying a rate the second quantity of working fluid is extracted from the storage tank responsive to a desired output of one or both of the work and the momentum.
 5. The method of claim 1, further comprising: monitoring pressure within the storage tank; and varying a rate the first quantity of working fluid is extracted from the storage tank to achieve and maintain a desired pressure within the storage tank.
 6. The method of claim 1, further comprising: monitoring temperature within the storage tank; and varying a rate the first quantity of working fluid is extracted from the storage tank to achieve and maintain a desired temperature within the storage tank.
 7. The method of claim 1, wherein the exothermically reacting operation includes one or both of decomposing and combusting the working fluid to generate the thermal energy.
 8. The method of claim 2, wherein the first quantity of working fluid is extracted from the storage tank primarily in a gaseous-phase and the second quantity of working fluid is extracted from the storage tank primarily in a liquid-phase.
 9. The method of claim 2, wherein the first outlet and the second outlet are located on opposing sides of the storage tank.
 10. The method of claim 1, wherein greater than 90% of the generated thermal energy is transferred into working fluid remaining within the storage tank.
 11. The method of claim 1, wherein the working fluid is a monopropellant.
 12. The method of claim 11, wherein the monopropellant has a vapor pressure greater than 101 kPa at 25° C.
 13. A storage tank comprising: a first outlet through which a first quantity of working fluid is extracted from the storage tank; and a heater that exothermically reacts the extracted working fluid generating thermal energy, wherein the heater further transfers a majority of the thermal energy into working fluid remaining within the storage tank.
 14. The storage tank of claim 13, further comprising: a second outlet through which a second quantity of working fluid is extracted from the storage tank, wherein the second quantity of working fluid is exothermically reacted to provide one or both of work and momentum.
 15. The storage tank of claim 14, further comprising: a first control valve that varies a rate the first quantity of working fluid is extracted from the storage tank responsive to a combined rate the first and the second quantities of working fluid are extracted from the storage tank.
 16. The storage tank of claim 15, further comprising: a second control valve that varies a rate the second quantity of working fluid is extracted from the storage tank responsive to a desired output of one or both of the work and the momentum.
 17. The storage tank of claim 13, further comprising: a pressure transducer that monitors pressure within the storage tank; and a first control valve that varies a rate the first quantity of working fluid is extracted from the storage tank to achieve and maintain a desired pressure within the storage tank.
 18. The storage tank of claim 13, further comprising: a temperature transducer that monitors temperature within the storage tank; and a first control valve that varies a rate the first quantity of working fluid is extracted from the storage tank to achieve and maintain a desired temperature within the storage tank.
 19. The storage tank of claim 13, wherein the heater includes: a decomposition/combustion chamber that performs one or both of decomposing and combusting the working fluid to generate the thermal energy; and a heat exchanger for transferring the majority of the generated thermal energy from the decomposition/combustion chamber to the working fluid remaining within the storage tank.
 20. The storage tank of claim 14, wherein the first quantity of working fluid is extracted from the storage tank primarily in a gaseous-phase and the second quantity of working fluid is extracted from the storage tank primarily in a liquid-phase.
 21. The storage tank of claim 14, wherein the first outlet and the second outlet are located on opposing sides of the storage tank.
 22. The storage tank of claim 13, wherein greater than 90% of the generated thermal energy is transferred into working fluid remaining within the storage tank.
 23. The storage tank of claim 13, wherein the working fluid is a monopropellant.
 24. The storage tank of claim 23, wherein the monopropellant has a vapor pressure greater than 101 kPa at 25° C.
 25. A system comprising: a working fluid storage tank with a first outlet and a second outlet; a heater fluidly connected to the first outlet that exothermically reacts working fluid discharged via the first outlet to generate thermal energy, wherein the heater further transfers a majority of the generated thermal energy into working fluid remaining within the storage tank; and an engine fluidly connected to the second outlet that exothermically reacts working fluid discharged via the second outlet to generate one or both of one or both of work and momentum.
 26. The system of claim 25, further comprising: a pressure transducer that monitors pressure within the storage tank; and a control valve that varies a rate the working fluid is discharged from the storage tank via the first outlet to achieve and maintain a desired pressure within the storage tank.
 27. The system of claim 25, wherein greater than 90% of the generated thermal energy is transferred into working fluid remaining within the storage tank.
 28. The system of claim 25, wherein the working fluid is a monopropellant.
 29. The system of claim 28, wherein the monopropellant has a vapor pressure greater than 101 kPa at 25° C. 